1. Field of the Invention
The present invention relates to a microscopic area analyzing apparatus and method for applying a primary electron beam to a surface of a solid sample to detect secondary electrons having a specific energy, released by the application of the primary electron beam, to perform an elemental analysis of a microscopic area of the surface of the solid sample.
2. Description of the Related Art
When a primary electron beam accelerated to several hundreds of electron volts to several tens of kiloelectron volts is applied to a surface of a solid sample, Auger electrons and characteristic X-rays, etc. are released which are associated with the element of the solid sample. Of these, the Auger electrons have an energy specific to the element. The AES (Auger electron spectroscopy) to measure the elemental composition of the surface of the solid sample by performing an energy analysis of the Auger electrons is an analyzing method providing high spatial resolution and surface sensitivity and finds applications in various fields. An apparatus where a function is added of scanning the sample surface with a micrified primary electron beam is the SAM (scanning Auger microscopy) apparatus. Prior arts concerning the AES and the SAM are disclosed, for example, in Japanese Unexamined Patent Publications JP-A 51-45585 (1976) and JP-A 51-7993 (1976).
FIG. 8 shows a basic structure of a conventional AES apparatus. An electron gun comprises a filament 15 for emitting a primary electron beam 8 and a lead wire 16. A focusing lens 17 is provided for focusing the primary electron beam 8. Deflecting electrodes 18 are provided for deflecting the primary electron beam 8. An objective lens 19 is provided for micrifying the primary electron beam 8. A detector 20 is provided for performing an energy analysis of the Auger electrons. A solid sample 21 to which the primary electron beam 8 is applied is fixed onto a sample support 22. A secondary electron detector 24 is provided for image observation.
As shown in FIG. 9, conventionally, the solid sample 21 is fixed onto a metal-made sample support 22 with sample fixing screws 5. In this case, as shown in FIG. 10, when the primary electron beam 8 accelerated to several hundreds of electron volts to several tens of kiloelectron volts is applied to the solid sample 21, the primary electrons are scattered and secondary electrons and characteristic X-rays are generated within the solid object based on the application of the electron beam, so that the Auger electrons are excited. In areas other than the area to which the thinned beam 8 is applied, the Auger electrons are also excited by the scattered electrons, the secondary electrons and the characteristic X-rays caused in the solid sample 21. By this process, of the Auger electrons excited in an Auger electron excitation area 23, only Auger electrons excited in an Auger electron generation area 9 which is smaller than the area where the Auger electrons are released (approximately several nanometers from the sample surface) are detected as signals.
Conventionally, the analysis of a microscopic area with the AES or the SAM apparatus is performed by micrifying the diameter of the primary electron beam 8. In recent years, an advance in the technology has enabled the diameter of the primary electron beam 8 to be micrified to approximately 100 angstroms so that an area 14 to which the primary electron beam 8 is applied is micrified. However, due to the influence of the scattering of the primary electron beam 8, etc. mentioned above, the Auger electron generation area 9 is several times greater than the actual diameter of the primary electron beam 8 as shown in FIG. 10.
FIG. 11 shows as an example of the solid sample 21 a sample of an LSI comprising a silicon oxide film 11, an aluminum film 12, a titanium-tungsten film 13 and another silicon oxide film 11. FIG. 12 shows results of an AES analysis of the aluminum film 12 of the solid sample shown in FIG. 11. As for the thicknesses of films of the solid sample 21, the aluminum film 12 is approximately 6000 angstroms thick and the titanium-tungsten film 13 is approximately 3000 angstroms thick. The AES measurement is performed by use of the primary electron beam 8 of 1 nanoampere. The acceleration at this time is made at 20 kiloelectron volts. The diameter of the primary electron beam 8 at this time is not more than 3000 angstroms.
As shown in FIG. 12, in addition to an Al-KLL Auger peak from the aluminum film 12 in the solid sample 21, a weak Si-KLL Auger peak is detected. Although the primary electron beam 8 is applied only to the aluminum film 12, a minute quantity of silicon is detected due to the expansion of the analysis area associated with the scattering of the primary electron beam 8 in the solid sample 21.
FIG. 13 shows a case where the analysis is performed with a thinned sample 1 fixed to the sample support 22. The thinned sample 1, which has a smaller thickness in its center, is fixed to the sample support 22 by use of the sample fixing screws 5 and a sample fixing plate 6. When the AES is performed, as shown in FIG. 14, the expansion of an Auger electron excitation area 23a in the thinned sample 1 can be restrained. However, a primary electron beam 10 transmitted by the thinned sample 1 excites the Auger electrons on the surface of the sample support 22 to form an Auger electron excitation area 23b. In addition, while in the analyzing method using the conventional thick solid sample 21, reflection of the primary electron beam 8 from the sample support 22 causes no problem because the scattering area of the primary electron beam 8 stays within the solid sample 21, in the analyzing method using the thinned sample 1, the primary electron beam 8 reflected by the surface of the sample 22 is again transmitted by the thinned sample 1.
Japanese Unexamined Patent Publication JP-A 51-45585 discloses a microscopic area analyzing apparatus using a thinned sample. In this prior art, in order to improve the focusing characteristic of the primary electron beam, a detector is disposed opposite to the side of the primary electron beam application to the thinned sample so as not to include the optical axis of the primary electron beam, and in the arrangement totally different from that of FIG. 8, the area to be analyzed is micrified. However, the primary electron beam transmitted by the sample is not removed. In addition, no consideration is given to the scattering and the reflection of the primary electron beam transmitted by the sample.
In Japanese Unexamined Patent Publication JP-A 51-7993 (1976), the sample support incorporates a Faraday cylinder and the sample current derived from the primary electron beam for scanning the sample surface is precisely measured. The Auger analysis is performed with the arrangement as shown in FIG. 8 similarly to the above-described prior art. Nothing is considered as to the micrifying of the analysis area for the Auger analysis.
In Japanese Unexamined Patent Publication JP-A 6-273297 (1994), a prior art is disclosed which uses a Faraday cup for detecting the endpoint in etching the sample with an ion beam. When the primary electron beam is applied to the sample surface while the sample is being etched with an ion beam, the primary electron beam is transmitted by the sample when the sample becomes sufficiently thin, so that the primary electron beam reaches the Faraday cup disposed on the rear surface of the sample. This enables the etching to be stopped when the sample has an appropriate thickness. However, the idea is not shown of performing a surface analysis of a microscopic area of the sample.